A typical packet-based communications network, such as the internet, allows users to communicate with each other using a communication channel in the network. The communication channel can be used to transfer speech signals between users in the network using a protocol such as the Voice over Internet Protocol (VoIP) as is known in the art. This allows the users to have a conversation with each other over the communications network. Speech signals are encoded with a codec at a first user terminal to compress the speech signals before they are transmitted over the communication channel to a second user terminal. At the second user terminal the speech signals are decoded with a codec to output the speech signals to the user. As is known in the art, the encoding and decoding processes include sampling the speech signal at a particular sampling rate. A greater sampling rate will generally result in a higher quality for the speech signal, but the network bandwidth required to transmit the signal will be increased.
The amount of data travelling over the network (i.e. the network load) will vary over time. The network bandwidth available in the network for a particular communication channel changes over time as a consequence of the varying network load as well as other time varying factors.
Some speech codecs, such as hybrid speech codecs, are able to switch between a set of available internal sampling rates. This allows the sampling rate used to encode and decode the speech signals to be dynamically adjusted in real time in dependence upon the current network bandwidth available in the communications network. In this way, the quality of the speech signal can be improved without exceeding the available network bandwidth of the communication channel. The hybrid speech codecs might switch the sampling rate immediately when a switch is desired. Alternatively, the codecs might wait to switch the sampling rate so that the switch is made during a period of speech inactivity. This ensures that the switch takes place when the speech signal is low so that the distortion in the frame in which the switch is carried out is low.
However, switching the sampling rate from a first sampling rate to a second sampling rate can cause a sudden change in the audio bandwidth of the speech signal. A sudden change in the audio bandwidth is noticeable in the speech signal and can be disturbing to the conversation. For the user receiving the speech signals, the sudden change in audio bandwidth is easily detectable and is perceived as a change in the characteristic of the speaker. The sudden change in audio bandwidth is particularly noticeable when the switch in internal sampling rate happens during a short period of speech inactivity, but during a period of high speaker activity, e.g. between two words in a sentence. Furthermore, when background noise is moderate or high, the switch in internal sampling rate will instantaneously change the characteristics of the background noise, thereby making the switch in sampling rates more noticeable in the speech signal.
The present invention has been made in the context of the prior art described above.